DIY Rack mount 1U JCM 800 preamp.

[dragonbat13]

Have been thinking about one of these for awhile. I ordered some of these a few days ago:

https://frogpedals.com/index.php/produc ... er-supply/

I just can't believe that little thing could suffice for a preamp like what's in the title.

I don't know anything, yet, about SMPS power supplies. What's the catch? I'm thinking noise and oscillation problems.

If this won't work I'll use some sort of torrid power transformer.

How does everyone feel about all this?

[martin manning]

SMPS generally operate at frequencies well above the audible range. In the link you posted 32kHz is mentioned. It might not be that high, but still over 20kHz.
You only need a few mA of high voltage supply current to run a preamp, about 1mA per triode, and in my experience that circuit can supply enough for two 12AX7's at reasonably high B+ voltage. The output voltage will drop with increased HV current demand. You'll need a robust DC supply to provide enough current for the SMPS and the heaters, and I'd recommend running it on 12V.

[xtian]

Here is a thread that will interest you. I built an SMPS powered "desktop" amp, and found no issues with noise from the SMPS.

https://ampgarage.com/forum/viewtopic.php?t=34374

[dragonbat13]

Here is a thread that will interest you. I built an SMPS powered "desktop" amp, and found no issues with noise from the SMPS.

https://ampgarage.com/forum/viewtopic.php?t=34374

Good info. Thanks. Thanks to you to Martin.

Here is the rack I'm considering:

https://www.budind.com/series/general-u ... es_group=2

Don't know it the link gives exact deminsions. The external dimensions would be 1.75x17x8. I want it shallow because all my other stuff fits in a shallow rack.

[Roe]

You only need a few mA of high voltage supply current to run a preamp, about 1mA per triode,

yes 1ma for ordinary gainstages and 3ma for the cathode follower, giving a total of 6ma for a 2203 preamp without phase splitter

[martin manning]

yes 1ma for ordinary gainstages and 3ma for the cathode follower, giving a total of 6ma for a 2203 preamp without phase splitter

1 mA per triode is an average number, good for quick estimates. In the 2204, the first stage is biased on the cool side at 0.835, and the second is the cold clipper, at 0.327. The third stage is a little hot, 1.51, and then there is the CF, which also biases hot, at 1.93. Those numbers are from a DC op point simulation, and they sum to 4.61 mA. Referring to Mark Huss' annotated 2204 schematic below, you can calculate the currents, but just looking at the dropping resistor from the PI node you can see the total preamp current there is 4.2 mA. Of course the triodes in the sim are on spec and identical, so its no surprise that the agreement isn't exact. I started at the 390V PI node, and the other voltages are close, within 5%. Using the SMPS above, the supply voltage won't be as high, so the current will be a bit less. If it can supply 300V at the CF anode, the total current will be just about 4 mA.

[dragonbat13]

Unfortunately, this supply will not provide 300 volts from what I see.

I didn't buy them specifically for this, just wanted to see if they would work.

[martin manning]

Unfortunately, this supply will not provide 300 volts from what I see.

I have a board with the same boost circuit (except its output cap is a 2u2), and it can do 285V at 4 mA running on 12VDC. That would put 205V on the input triode's plate, which would be acceptable, I think.

[dragonbat13]

What about the 300 volts on the cathode follower?

I'm going to read up more on these power supplies.

[martin manning]

Connect the CF directly to the SMPS.